a lot of the variables are all intertwined in relation. speed depends on the wake height, boat type, skill of rider, and the length of the rope itself and all to one another. Most of all it is going to depend on the boat type and the wake it puts out
The longer you can have the rope out the better. This will push you to edge better and pop harder. more importantly it will give you more time to perform tricks that you will be learning. But with this said it is best to have the rope at the farthest length just before the wake begins to lose its smooth shape and wash out or roll over on itself.
I would like to say that the boat speed should be no slower than 20mph. (I personally wakeboard at about 22mph). Also recognize that the faster the boat goes the more the wake will be "crisp," putting the washed out portion further back. the wake will naturally narrow up a bit too.
I feel comfortable stating that you and anyone else should be riding with a rope length no shorter than 60ft... and this would only be temporary in starting maybe.
I don't have a whole lot of experience with weighing a boat down, but additional weight is something that is to be considered. check this link here http://www.wakeworld.com/tricklist/tricktip029.asp